VFD failure?

Hi all, has anyone ever seen a VFD (vacuum fluorescent display) failure in which half the display is very dim and the other half looks good? I have this 16 character (16 segments) 1 row display and the first 8-10 characters have a normal brightness, the rightmost ones are very dim, barely readable. I first suspected a problem with the filament supply, but swapping the filament pins has no effect, so I'm a bit puzzled. Waveforms look identical on all the 16 grids too. Unfortunately I have no schematic of this instrument (digelec 824 eeprom programmer), nobody else in the world seems to have this programmer also, so I could not ask anyone for some quick waveform comparison. I'm starting to suspect a bad VFD but I can't imagine how would it fail like this. Part of the filament supply is made with a NE555 that's getting quite hot and also a 120 ohm 1/4W resistor near it gets too hot to touch, but all components I could test looks ok. I swapped the 555 and it doesn't make any change. If I leave the 555 out, the display has no filament supply. Any hint is really welcome. Regards

Frank IZ8DWF

Reply to
frank
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On Wed, 7 Sep 2016 07:38:16 +0000 (UTC) frank wrote in Message id: :

Just a WAG, but perhaps in normal operation the dim digits were constantly lit, while the bright ones weren't?

Reply to
JW

could be, but I doubt it. The brightness starts going down from digit 10 and it is really progressive, each digit is dimmer than the one on its left. Things without a schematic are difficult sometimes. What really puzzled me is the fact that swapping the filament pins didn't make any difference. I would suspect the driver (Rockwell 10937), but grid waveforms look all the same. Frank

Reply to
frank

On Wed, 7 Sep 2016 12:09:50 +0000 (UTC) frank wrote in Message id: :

Interesting. Under a bright light can you see if the phosphor for each digit incrementally looks darker for the dimmer digits?

I've never seen that fault before, but I have noticed that when VFDs get dim the phosphor looks darker than a new one.

Reply to
JW

The phosphor looks identical on all digits, as far as I can tell.

I'm going to try to swap the last and first grid connection, that should give me some more hints (I hope).

Frank

Reply to
frank

swapping the grids leave the bad digits at the same physical place, of course the displayed characters swap too. This is another point in favour of bad VFD, which is unobtanium anyway, it's a futaba 16-SY-03Z.

Frank

Reply to
frank

On Wed, 7 Sep 2016 13:16:38 +0000 (UTC) frank wrote in Message id: :

I think you're right. That's a real weird failure mode, though. Not unobtanium.

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but ridiculousium.

Any reason you can't move to a newer programmer? You're not programming

2708s or anything?
Reply to
JW

yeah, the programmer costs much less

Actually I'm programming a lot of old things, including MCM68764, 2716, most of the MCUs that this old programmer supports... You would be surprised to discover how few programmers support (correctly) the 2532s and MCM6876x (or ricoh pin compatible parts) for example. Anyway... I can live with a magnifying glass for the moment. Frank

Reply to
frank

That would suggest to me that the filament is being driven by DC, rather than by AC or a square wave.

The filament is the cathode in the tube. The voltage difference between the cathode, and the fluorescent anode sections, greatly affects the brightness. If you have a voltage drop across the width of the filament (with one side grounded) you'll have different cathode-to-anode voltages across the width of the display, and different digit brightnesses.

One way to avoid this (the usual way) is to heat the filament with an AC or square-wave-DC signal, typically symmetrical around ground. This results in the same average voltage between the anodes, and different points on the filament, and evens out the brightness. If you drive the filament with AC from a separate transformer winding, it's necessary to create some sort of ground return reference for it (for the cathode-to-anode electron flow). This can be done by grounding a center tap on the transformer secondary, or grounding a center point on the filament itself.

The fact that swapping the filament pins didn't affect the problem, makes me suspect that there's a fault inside the tube. Possibly there's an internal short between one end of the filament, and ground.

So, take a look at the voltages and waveforms on the filament drive pins. Ideally, they should be exactly mirror-opposites of one another, with respect to ground. I suspect you'll find that they are not.

Reply to
Dave Platt
[...]

I haven't been able to measure any short between filament and the rest of the internals (grid, anodes). I've also spent some time checking for shorts between grid-grid, grid-anodes and even anodes-anodes. Since characters are generally made correctly, I didn't expect any short, but I was out of ideas.

That's the first thing I checked actually. The voltages are not symmetrical respect to the ground, so I swapped the filament pins expecting to have an opposite behaviour, but, the filament supply didn't change (it did not follow the pin swapping) and the brightness situation of all the characters didn't change. The filament waveform is a square wave with rather low duty cycle (and this also probably explains why they got away with this non symmetrical supply in the first place), the DC component is about -11V and it is present on both filament pins, and the square pulses go to about -21V and are applied on one side of the filament. It is obtained with a NE555 based circuit, the duty cycle is probably ok as I measured an RMS value of 6V on the filament and this particular VFD wants 5.8V nominally. I can't still exclude a rather obscure (at least to me) problem in the filament supply, but components are very few and they are all ok. This instrument is rather large with more than 100 ICs scattered around, so I'm not going to reverse engineer a schematic out of it in my lifetime. All the tests in my opinion points to lazy phosphors or low emission from the filament in the last digits. If it was the asymmetrical filament supply, the dim digits would follow the pin swap of the supply. Maybe the hypothesys that the last digits were the ones most used isn't that bad after all. VFD are really nice looking displays, but I must remember to avoid old instruments with those :) Frank

Reply to
frank

You might take a look at this:

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Reply to
tom

If the filament were made up of two parts in parallel, and one of the parts failed open, could that account for both the dimness and the overheating 555?

Take the thing into a dark room; often you can see a dim glow from the filament. Do you see it over all parts of the display, or only over the "normal" part?

Isaac

Reply to
isw

the filament has two wires that run for all the display length. However I can't tell if these two wires are in parallel or in series electrically. Supposing that the two wires are in parallel, then since they run for all the display length, it shouldn't make any difference on only the rightmost digits. One open wire would also mean higher resistance so probably the 555 would run cooler too.

I'll try to verify this, just for the sake of it.

Frank

Reply to
frank

I dunno - I've seen many VFD's that simply were aged and had similar problems.

Replacement was the fix. Increasing the filament voltage might help, but I suspect the dimmer segments still won't be as good.

I once fed about 60 volts to a nominal 5 volt filament before it failed, so I don't think they are terribly fragile.

Mark Z.

Reply to
Mark Zacharias

Hello all,

I've seen several VFD's that have this issue, usually on the segments that are rarely turned on, like a 12 digit calculator. One temporary fix is to type in a ll "8" on the display, to light the greatest number of segments and let it burn in that way for 24 hours. That usually fixes the problem for a few months.

Regards, Tim Bristol Electronics

P.S. One amplifier manufacturer I deal with and had problems with their display issues a new software revision that had a "light everything on the display" function which they instructed you to leave on for 24 hours, and it did work. I think contamination plates rarely used segments.

Reply to
Tim Schwartz

Thanks for the informations, sounds interesting. I have to make some experiments on this display, either using the driver or making a temporary circuit to light all segments. It's 16 digits (characters really) with

16 segments each.

Frank

Reply to
frank

Interesting. I don't think this would help with some dot-matrix type VFD's like the newer Yamaha's, where a few I've seen will light every other row unevenly, or like a heavily used Fluke 8840 with each segment lit more in it's middle and dimmer at each edge, but seeing as how Yamaha has a "light all" option in their diagnostic menu I plan to keep your post here for posterity. Might come in handy (or not - I'm retiring soon; don't know how much I'm going to be doing audio in the future)

:-(

Mark Z.

Reply to
Mark Zacharias

Is the DC bias on the filament a nice flat line on the scope ?

Reply to
jurb6006

yes. Looking better, some segments all over the places are dimmer than the others. Looks like this display is at the end of its useful life. I can't tell if the dim segments were the most used or most unused ones. Probably a lot depends on how the previous owner(s) used this thing. Frank

Reply to
frank

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